Juno was Jupiter's wife, but he fooled around a lot, and had to turn human girls into swans when Juno unexpectedly showed up. The magnetosphere is the zone surrounding Jupiter which contains Jupiter's magnetic field. Jupiter's magnetic field is intense, powerful, so the magnetosphere extends far from the planet itself.

(Juno will go into orbit around Jupiter on 4 July. I don't know if some Flag-Simple nerd planned it that way, or if it's just an accident of the necessary mechanics of sending robots to other planets. Probably it's just a coincidence.)

Juno sensed this event in several sensitive ways (but the fundamental new sensitivity is that we've never sent a machine this close and intimately to Jupiter before), sent the data back to Earth, and computers and their pet boffins turned it into music. Music from Jupiter of Juno's encountering Jupiter's magnetosphere. Click on the video at post top to hear the tune.